1. Field of the Invention
The present invention is related to thermal oxidation processes at low temperatures These processes can be used during the manufacturing of semiconductor devices. Specific examples of use of such processes are the growth of thin oxide layers down to 0.1 nm, the Cl-cleaning of a substrate and the temperature ramp-up prior to the oxide growth during a rapid thermal oxidation process.
2. Description of the Related Technology
In thermal oxidation processes an aim is to grow SiO.sub.2 films by exposing silicon to O.sub.2 at elevated temperatures. Historically chlorine has been introduced in the oxidation ambient in order to improve the electronic quality of gate oxide layers. Studies have revealed that the improvements by introducing chlorine are in fact initiated by the presence of Cl.sub.2. Particularly, the reduction of electronic instabilities, attributed to the presence of mobile ions, mainly Na, has been emphasized. In addition, the use of Cl during gate oxidation was also found to result in a reduction of the density of dielectric breakdown defects and of stacking faults. It has been demonstrated that metal contamination on the wafer surface prior to gate oxidation has a distinct negative effect on the dielectric integrity of thin oxides. Particularly Ca has been identified as one of the most detrimental metals in that respect. The introduction of Cl in the oxidation ambient was found to be very efficient in removing metal contaminants, especially Ca, from the silicon wafer surface. In order to meet the stringent future gate-oxide defect density requirements, the residual concentration of metals and of Ca in particular has to be further reduced.
Most oxidation tools are now equipped for the introduction of chlorine species during silicon wafer oxidation and/or in situ tube cleaning operations. Several precursors have been used to introduce chlorine. In order to compare these different methods a common parameter describing the concentration of the total amount of Cl fed to the reactor chamber, irrespective of its chemical state, is introduced. Said parameter is the "chlorine-equivalent concentration of a given Cl-precursor" and is defined as the ratio between "the total flow of Cl atoms [number of Cl atoms per unit time] to the process chamber" and "the total flow of all molecules [number of molecules per unit time] to the process chamber".
In the past it was common practice to feed HCl gas to the oxidation furnace. Although this gas was effective for this application, its use has several drawbacks. Because of its corrosive nature, this gas deteriorates the metal distribution lines as well as the metal components in the gas management system. Such corrosion phenomena result in highly undesirable metallic contamination of the gasses. Moreover the handling of the pressurized gas cylinders requires special care.
Because of these drawbacks the industry has used 1,1,1-trichloroethane (TCA) as the precursor for Cl in the furnace. TCA is a volatile liquid and can be introduced into process tools via Teflon.TM. tubing thereby avoiding the corrosion phenomena faced with HCl. Since TCA has been identified as an ozone depleting substance, attacking the stratospheric ozone layer, its production, use and/or transportation has been restricted or even banned.
In response the industry has come up with ozonelayer-friendly replacement substances for TCA such as trans-1,2-dichloroethylene (DCE) and oxalyl chloride (OC). The replacement with DCE is the subject of the U.S. Pat. No. 5,288,662. The replacement with OC is the subject of the European Patent EP 0 577262 B1. However for instance U.S. Pat. No. 5599425 clearly states (column 1, lines 10-35 and especially lines 33-35) that organic chlorine precursors are typically not used as chlorine precursors in processes carried out at temperatures below 800.degree. C. U.S. Pat. No. 5599425 suggests the predecomposition of these organic molecules in a pre-burn box.
The ongoing downscaling of CMOS device dimensions, in particular the gate length, demands for an ongoing reduction of the gate oxide thickness in order to meet the required stringent device performance specifications. The realization of this required shrinkage of the thickness of oxide layers with the conservation or preferably even improvement of the quality of these oxide layers, causes severe problems. Amongst others, to obtain a high quality oxide layer, the metal contamination level should be as low as possible.
B. -Y. Nguyen et al, in Tech. Dig. 1993 Symp. on VLSI Technol., (JSAP, Tokyo, 1993) p. 109 is related to a so-called "pyro-clean" process. In this process an in-situ low temperature Cl-treatment prior to the gate oxidation process is used. The motivation for this process is based on the fact that the diffusion constant and the solubility of a number of metals in silicon increases strongly with increasing temperature. The purpose of this process is to remove metallic contamination before the onset of diffusion of metal into bulk silicon. Typically a 30 minutes treatment at 650.degree. C. is performed using an inert (e.g. N.sub.2) ambient containing O.sub.2 at a volume concentration of 2%. As a Cl precursor, HCl was chosen. As mentioned above, the use of the corrosive gas, HCl, in this process leads to potential danger of corrosion of the gas distribution system and is therefore undesirable.
U.S. Pat. No. 5480492 is related to the removal of contaminants from the silicon substrate surface by exposure to a gas including ozone at a temperature not less than 750.degree. C. However, the conditions are such that oxidation of the silicon substrate surface is prevented, in other words no oxide layer is formed.
U.S. Pat. No. 5589422 is related to the removal of metal contaminants from the silicon substrate surface by exposure to an ozone ambient. An oxide layer is formed and subsequently removed prior to the formation of a structural oxide layer.
U.S. 5330935 is related to the formation of silicon dioxide films by lowtemperature plasma oxidation employing a mixture of oxygen and ozone. However, as stated in column 1, lines 25-30 of this paten, it is known that thin silicon dioxide films formed by thermal oxidation have a higher quality as films formed by plasma oxidation. U.S. Pat. No. 5330935 further states that contemporary state-of-the-art thermal oxidation processes are limited to the high temperature regime, i.e. typically in excess of 900.degree. C.